DESCRIPTION (appended verbatim from investigator's abstract): The objective of this proposal is to determine the mechanism of LKB1/STK11, a serine/threonine kinase, in mediating apoptosis. The mutations in LKB1 are responsible for majority of Peutz-Jegher syndrome cases (PJS; MIM 175200), an autosomal dominant disease characterized by melanocytic macules of the lips, multiple benign gastrointestinal hamartomatous polyps in early life and a dramatically increased risk for various cancers later in life. The discovery that the PJ wild-type allele was lost in the hamartomas has led to the suggestion that the target of the deletion was a tumor suppressor gene. LKB1 is the first and so far the only kinase whose loss-of-function mutations predispose to tumorigenesis, which makes it very interesting to explore its mechanism of action. Through our preliminary studies, we found that LKB1 may play an important role in regulating apoptosis. Overexpression of wild type LKB1 but not a kinase dead mutant induces apoptosis. Endogenous LKB1 is present both in cytoplasm and nucleus; upon induction of apoptosis by paclitaxel or Fas, endogenous LKB1 is translocated to mitochondria. The LKB1 kinase domain alone induces apoptosis and translocates to the mitochondria more efficiently than that of wild type, whereas the kinase dead LKB1 mutant can neither induce apoptosis nor translocate to mitochondria, suggesting that both apoptosis and mitochondrial translocation require its kinase activity and the regulatory domain of LKB1 plays a negative role in regulating its apoptotic activity. Furthermore, a dominant negative mutant of LKB1 inhibits apoptosis induced by paclitaxel and to a weaker extent by etoposide but not that by Fas. We propose that LKB1 is an important apoptotic signal transducer and is required for inducing apoptosis through mitochondrial damage through a p53-dependent mechanism. This proposal is to determine the functional domains of LKB1 required for induction of apoptosis in HT1O8O cells and in the small intestine epithelium cells. We propose that autophosphorylation plays an important role in negative regulation of LKB1 activity. We will determine the specificity of LKB1 mediated apoptotic pathway using the dominant negative LKB1 mutant. To illustrate the apoptotic pathway mediated by LKB1, we will identify the downstream targets of LKB1 in mitochondrial translocation. To establish the in vivo function of LKB1, we will examine chimeric transgenic and transgenic mice expressing the dominant negative LKB1 mutant using an intestinal epithelium-specific promoter. We will also search for evidence of apoptosis inhibition in the polyps regions of the intestinal samples from Peutz-Jegher patients.